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Wednesday, November 2, 2022
Question:
Good morning CQD, what is the largest size circuit breaker that can be used to protect 8 gauge and 6-gauge Romex (NM) Cable. The NM Cable will be installed in a residential basement. This information would be helpful for wiring hot tubs and car chargers. Thanks, Kent Taylor.
A
Answer:
Hello Kent, thanks for participating in the CQD, and well this seems like a straightforward question, it has more to it. Article 334 of the NEC® provides guidance on the installation of NM Cable. Section 334.80 provides additional rules for the ampacity of NM Cable. In general, the ampacity of NM Cable cannot exceed that of a 60° rated conductor. Taking this rule to the 60° column of Table 310.16, the maximum ampactiy for #8 AWG copper conductor in the NM Cable would be 40A. Applying the same process for a #6 AWG copper conductor in the NM Cable, the maximum ampacity would be 55A.
To determine the maximum size circuit breaker (OCPD) we must go to Section 240.6(A) for standard size fuses and circuit breakers. The 40A maximum ampacity for a #8 NM Cable aligns with the standard size 40A circuit breaker. The 55A maximum ampacity for a #6 AWG NM Cable
does not align with a standard size circuit breaker. Section 240.4(B) permits going to the next higher standard circuit breaker, when the ampacity of the conductor does not align with the standard sizes. The conditions found in Section 240.4(B)(1)-(3) must all be met to move up to the next standard size. In this case all 3 conditions are met. The maximum circuit breaker for a #6 AWG NM Cable would be 60A.